About constant modifier const

      Has been to const Is a simple constant modifier , Define an immutable variable 【 Notice that here are variables , in other words const Called constant modifier , But in fact, it defines a variable , It is only limited by the compiler and cannot be modified 】.

        After listening to the teacher's class, I found const There are many interesting uses ：

        1：const int i = 10; --> i The value of cannot be changed , if i = 11; False report

        2：const int *j = (int*)&i; and int const *j = (int*)&i; --> j The content of the variable pointed to is immutable , if *j = 20; False report

        3：int * const t = (int*)&i; --> t The address pointed to cannot be changed , if t++; False report

        The teacher's advice is to see const What's ahead , Then that part cannot be modified , such as int const i = 5; const Before is int, said i Cannot be modified

                                                                                                                                  int const * j; const Before, too int, said i The value of the variable pointed to cannot be modified

                                                                                                                                  and int * const j; const Before is int *, Indicates that the value pointed to by the pointer cannot be modified

        There is another interesting one that you can try by yourself , I don't know why ,

const in a = 10;
int *p = (int*)&a;
int *j = (int*)&a;

cout << *p << " " << a <<  " " << *j << endl;
	cout << p << " " << &a <<  " " << j << endl;
	*p = 20;
	cout << *p << " " << a <<  " " << *j << endl;
	cout << p << " " << &a <<  " " << j << endl;

The result of the above code is very interesting , After execution *p = 20; after ,*p and *j All become 20, however a still 10. At the same time, you can see p and j Address to and a The address of is the same .

I found the answer in others' blogs

http://blog.chinaunix.net/uid-26853826-id-3365403.html

const Global variables are stored in the global storage space , Its value is only readable , Do not modify ;

const Local variables are stored on the stack , Its value can be modified through the pointer ;

const Variables are processed in preprocessing , The compiler reads its value only once .